These are to prevent the extent of the untruncated span from xl to xu and yl to yu from being unable to fit in the hash bit allocation (6 bits each).

In the case of width, in the case where the lower 9 bits of (l - (70 * ZOOM_LVL_BASE)) are greater than the lower 9 bits of r,
dpi->width + (70 * ZOOM_LVL_BASE) can be less than (1 << (7 + 6 + ZOOM_LVL_SHIFT)) even when the the untruncated hash values differ by 0x40, such that the truncated values are the same.
This has the effect that if the viewport re-draw area is the right size and offset, the iteration over the vehicle viewport hash only finds vehicles at the two edges but not in the middle.
This can visually manifest as flickering effects.